The present invention relates to a space diversity reception system, in particular, relates to such a system which can reduce the amplitude dispersion and the delay dispersion caused by the frequency selective fading within the pass-band.
In a microwave FM (frequency modulation) circuit, a microwave from the transmission antenna ANT.sub.1 which is connected to the transmitter TX reaches the reception antenna ANT.sub.2 which is connected to the receiver RX through at least a direct path P.sub.1 and an indirect path P.sub.2 (see FIG. 1). The microwave through an indirect path is called an interference wave, and the phase between the direct wave and the interference wave changes at random. Supposing that the direct wave is A.sub.0 =a.sub.0 sin .omega.t, and the interference wave is A.sub.1 =a.sub.1 sin .omega.(t-.tau.), then, the attenuation of the combined wave is up to 20 log (1/(1-.psi.))(dB), (where .psi.=a.sub.1 /a.sub.0).
In order to combat said attenuation, a space diversity system which combines two signals received by a plurality of spaced antennas with inphase has been used.
FIG. 2 shows the block diagram of a prior diversity system, and FIGS. 3A, 3B and 3C show the operation of a prior diversity system.
In FIG. 2, the reference numeral 1 is the first antenna, 2 is the second antenna, 3 is a phase shifter, and 4 is a combiner. Supposing that the antenna 1 receives the main wave M.sub.1 and the interference wave I.sub.1 with the phase difference as shown in FIG. 3A, then, the reception power C.sub.1 is the vector sum of said M.sub.1 and I.sub.1 as shown in FIG. 3A. Similarly, supposing that the second antenna 2 receives the main wave M.sub.2 and the interference wave I.sub.2 with the phase difference as shown in FIG. 3A, then the reception power C.sub.2 is the vector sum of said M.sub.2 and I.sub.2 as shown in FIG. 3A.
In the prior diversity system, the vector sum C.sub.1 and C.sub.2 are combined at combiner 4 so that they are inphase with each other by adjusting phase shifter 3, and the resultant wave is C.sub.1+2.
The above mentioned prior diversity system intends to provide the maximum combined power, therefore, the prior diversity system is merely effective for the improvement of the signal-to-noise ratio of a narrow band signal.
However, said prior inphase combining diversity system can not always improve the frequency characteristics in the wide band trasmission system. In particular, when there are interference waves which transmit through a different length of paths, the frequency characteristics are deteriorated. Thus, a high speed digital radio system which has the wide band width and the high speed data information of for instance 50 MB, cannot be composed by a prior diversity system. The situation will be described in detail mathematically.
Assuming that an intereference wave and a direct wave are received by each antenna as shown in FIG. 2, then, the received signal by a single antenna, and the combined signal by two antennas are shown below.
(a) In case of reception by a single antenna EQU e.sub.i (.omega.)=l+re.sup.-j.omega..tau..sbsp.i =R.sub.i (.omega.)e.sup.j.theta..sbsp.i ( 1)
wherein i=1 or 2 (the suffix 1 relates to the first antenna 1, and the suffix 2 relates to the second antenna 2) ##EQU1## (b) In case of inphase combined reception by two antennas EQU X.sub.s (.omega.)=A.sub.SD .times.{l+De.sup.-j.omega..tau..sbsp.s }(4)
where ##EQU2## EQU .psi.=.theta..sub.1 -.theta..sub.2 (Condition of inphase combination) (6) ##EQU3## EQU .tau..sub.s =(.tau..sub.1 +.tau..sub.2)/2 (8)
r; the amplitude ratio between the main wave and the interference wave. PA1 .tau..sub.i ; the time difference between the main wave and the interference wave at the antenna (i). PA1 .omega.; =.omega..sub.0 +.DELTA..omega. PA1 .omega..sub.o ; the center frequency of the pass-band PA1 .DELTA..omega.; deviation from .omega..sub.o. PA1 .psi..sub.i ; the phase difference between the main wave and the interference wave caused by .tau..sub.i, and is .psi..sub.i =.omega..sub.o .tau..sub.i -2 N.pi..
FIG. 3C shows the calculated curves of the above analysis, where the curve (a) shows the frequency characteristics of antenna 1 output signal, the curve (b) shows the one of antenna 2 and the curve (c) shows the inphase combined signal. The conditions for the calculation in FIG. 3C are shown below. ##EQU4##
From FIG. 3C, it is noted that the curve (c) is a little more improved compared with the curves (a) and (b), but said curve (c) is not sufficiently flat. If the wide band digital radio signals are transmitted under the characteristics as shown in the curve (c), the waveform of the signal would be deteriorated, and the bit error rate would be increased.
Thus, it should be concluded that a prior diversity system is not sufficient to provide the flat frequency characteristics although said prior diversity system can improve the received signal level.